One thing to remember when backpacking (as opposed to day hiking) is that most of the "essentials" are part of your normal backpacking gear! In most cases there is no need to take separate items.

One thing I have learned to take is backup for the water filtration system, which in my case is a package of chlorine dioxide tablets. I learned the hard way last summer when I forgot the tablets and my Sawyer Squeeze filter was completely blocked and didn't respond to backflushing. Fortunately, I had extra fuel along (for frying fish that weren't caught), but boiling drinking water for two is a lot of work! In addition, I'll be testing the Sawyer Squeeze the night before I leave instead of several weeks before!